[tree] Also enable pre-fetching if there are no registered clusters
[tree] Also enable pre-fetching if there are no registered clusters

Before this commit, when TTree::fAutoFlush == 0 **and fNClusterRange ==
0**, the TTree cache size was set to 0, effectively disabling
pre-fetching.

With this patch we instead set the cache size to a reasonable default
also in this case, assuming the default cluster size of ~30MB.

This fixes #8713 also for smaller files where fNClusterRange == 0.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root